1. Field of the Invention
The present invention relates to a method for charging a secondary battery.
2. Description of the Related Art
Methods for charging a secondary battery can be generally classified into constant-voltage methods and constant-current methods. The constant-current method is capable of rapid charging but is likely to result in overcharging. The constant-voltage method generally controls the charge voltage at a level which is equal to or less than a voltage which causes hydrogen gas generation within the secondary battery. As a result, the charge current decreases as the charging process proceeds, thereby minimizing overcharging.
Thus, the constant-voltage method minimizes overcharging but may result in undercharging. Therefore, methods have been used which combine both the constant-voltage method and the constant-current method.
In the case of combining both the constant-voltage method and the constant-current method, the control voltage, current, and charge time are prescribed so that the proper charge electricity amount equals about 105% to about 120% of the discharged electricity amount.
In recent years, lead-based secondary batteries have replaced conventional liquid-type lead secondary batteries as power sources for various cycle services such as electric automobiles. In particular, lead secondary batteries of a sealed type, which absorb gaseous oxygen generated within the batteries at the negative plates by employing a limited amount of electrolytic solution, have been in use.
Sealed-type lead secondary batteries for cycle services may be subjected to various loads depending on the specific device for which they are used, and various usage time and/or frequency depending on the user. Therefore, the degree of discharge or "discharge depth" which is experienced by sealed-type lead secondary batteries may vary from battery to battery.
It has been discovered that, in the case of aforementioned sealed-type lead secondary batteries for cycle services, merely prescribing a certain charge electricity amount relative to the discharged electricity amount does not allow the sealed-type lead secondary batteries to exhibit desired longevity characteristics.
For example, it to conceivable that a sealed-type lead secondary battery which has not been well-discharged (i.e., "shallowly discharged") may be charged by using a charger which is intended for a well-discharged (i.e., "deeply discharged") sealed-type lead secondary battery. Such a charger has a relatively high charge voltage. In such cases, the lifetime of the sealed-type lead secondary battery may be drastically shortened even is a ratio of discharged electricity amount to charge electricity amount within a conventionally acceptable range is prescribed. The problem of such shortened lifetimes becomes especially conspicuous with sealed-type lead secondary batteries employing a Pb--Ca--Sn type alloy (not containing any Sb) as a positive grid alloy and employing a limited amount of electrolytic solution.
Moreover, the above-mentioned problem may unpredictably occur or may not occur at all, depending on the manner in which a given device associated with such a sealed-type lead secondary battery is used by a user. It is practically impossible to select different types of chargers depending on the manner in which a device associated with such a sealed-type lead secondary battery in used by a user.